ci: compile AllPlugins on push only when plugins change

[datlechin]

Why

Every push to main or release/** set up Go, built both native bridges and compiled AllPlugins, about 4 minutes of a macOS slot, even when no plugin input changed. PRs already skip that through the plugins detector.

Change

• macOS Tests compiles AllPlugins on a push only when the plugins detector is not false, as on a PR. A dispatch and a failed changes job still compile.
• The plugins detector also watches generate-project.sh, download-libs.sh, create-openssl-dylibs.sh, prepare-build and setup-xcodegen, which the plugin build reads. This closes the same gap on PRs.
• ci.md: macOS Tests, the detector and What runs where.

Verified

test_detect_changed_paths.py: the new plugins cases fail on main's detector for those 5 paths and pass here. actionlint, shellcheck and agent-docs pass.

Not in this PR

• A green macOS Tests run no longer means AllPlugins compiled at that commit. Release Build still compiles every registry plugin on each code push.